If someone didn’t learn enough trigonometry in school:
1+tan2c = cos2c/cos2c + sin2c/cos2c = (cos2c + sin2c)/cos2c = 1/cos2c;
sqrt(1/cos2c) = |sec c|
The reading of the answer is very similar to the word “sexy”, which makes the whole sentence a reference to the song “Sexy and I Know It” by LMFAO.
Well, on the side of easy ones there is “if the last digit is divisible by 2, whole number is divisible by 2”. Also works for 5. And if you take last 2 digits, it works for 4. And the legendary “if it ends with 0, it’s divisible by 10”.
The divisability rule for 7 is that the difference of doubled last digit of a number and the remaining part of that number is divisible by 7.
E.g. 299’999 → 29’999 - 18 = 29’981 → 2’998 - 2 = 2’996 → 299 - 12 = 287 → 28 - 14 = 14 → 14 mod 7 = 0.
It’s a very nasty divisibility rule. The one for 13 works in the same way, but instead of multiplying by 2, you multiply by 4. There are actually a couple of well-known rules for that, but these are the easiest to remember IMO.
Try MusicBrainz Picard. I’ve had good experience with their recognition quality.
I only remember useful things. And I consider these useful because I did some low level graphics programmed once and had to use them in practice. Once you understand why these things exist, it becomes that much easier.